I have a 2gm20f with a curious problem....the aftmost pushrod is intermittent....could this be a stuck lifter? Or would you think it may be a sticking valve? It figures that it is one of the coldest days of the year here, I maybe be suffering from "brain-freeze"

Just an update....Set the valves to proper clearance.....Did compression test......way low........did notice a hissing sound when slowly turning engine over (by hand)while tester was in aft cylinder........sound seemed to come from front hole.......Rigged up a leakdown adaptor.....with cyl #2 @TDC you could hear air from #1 cylinder.